Sparky Anderson began his professional baseball career as a second baseman in the Dodger organization. Signed by Brooklyn as a teenager, Anderson’s first pro season was in 1953 at age 19 for the Dodger minor league C-Level team.
Here's a letter written by a 20-year old Anderson to longtime Dodger executive Fresco Thompson in 1954. Anderson asks about the Colombian Winter League that’s due to start in mid-October. The youngster writes about his eagerness to make arrangements for a passport for travel. Three months shy of his 21st birthday, Sparky has signed his full legal name, “George Lee Anderson”.
Seems like an earlier example of his signature would be tough to find.